A mode is a set of definitions that customize Emacs and can be
turned on and off while you edit. There are two varieties of modes:
major modes, which are mutually exclusive and used for editing
particular kinds of text, and minor modes, which provide features
that users can enable individually.

This chapter describes how to write both major and minor modes, how to
indicate them in the mode line, and how they run hooks supplied by the
user. For related topics such as keymaps and syntax tables, see
section Keymaps, and section Syntax Tables.

Major modes specialize Emacs for editing particular kinds of text.
Each buffer has only one major mode at a time.

The least specialized major mode is called Fundamental mode.
This mode has no mode-specific definitions or variable settings, so each
Emacs command behaves in its default manner, and each option is in its
default state. All other major modes redefine various keys and options.
For example, Lisp Interaction mode provides special key bindings for
LFD (eval-print-last-sexp), TAB
(lisp-indent-line), and other keys.

When you need to write several editing commands to help you perform a
specialized editing task, creating a new major mode is usually a good
idea. In practice, writing a major mode is easy (in contrast to
writing a minor mode, which is often difficult).

If the new mode is similar to an old one, it is often unwise to modify
the old one to serve two purposes, since it may become harder to use and
maintain. Instead, copy and rename an existing major mode definition
and alter the copy--or define a derived mode (see section Defining Derived Modes). For example, Rmail Edit mode, which is in
`emacs/lisp/rmailedit.el', is a major mode that is very similar to
Text mode except that it provides three additional commands. Its
definition is distinct from that of Text mode, but was derived from it.

Rmail Edit mode is an example of a case where one piece of text is put
temporarily into a different major mode so it can be edited in a
different way (with ordinary Emacs commands rather than Rmail). In such
cases, the temporary major mode usually has a command to switch back to
the buffer's usual mode (Rmail mode, in this case). You might be
tempted to present the temporary redefinitions inside a recursive edit
and restore the usual ones when the user exits; but this is a bad idea
because it constrains the user's options when it is done in more than
one buffer: recursive edits must be exited most-recently-entered first.
Using alternative major modes avoids this limitation. See section Recursive Editing.

The standard GNU Emacs Lisp library directory contains the code for
several major modes, in files including `text-mode.el',
`texinfo.el', `lisp-mode.el', `c-mode.el', and
`rmail.el'. You can look at these libraries to see how modes are
written. Text mode is perhaps the simplest major mode aside from
Fundamental mode. Rmail mode is a complicated and specialized mode.

The code for existing major modes follows various coding conventions,
including conventions for local keymap and syntax table initialization,
global names, and hooks. Please follow these conventions when you
define a new major mode:

Define a command whose name ends in `-mode', with no arguments,
that switches to the new mode in the current buffer. This command
should set up the keymap, syntax table, and local variables in an
existing buffer without changing the buffer's text.

Write a documentation string for this command that describes the
special commands available in this mode. C-h m
(describe-mode) in your mode will display this string.
The documentation string may include the special documentation
substrings, `\[command]', `\{keymap}', and
`\<keymap>', that enable the documentation to adapt
automatically to the user's own key bindings. See section Substituting Key Bindings in Documentation.

The major mode command should start by calling
kill-all-local-variables. This is what gets rid of the local
variables of the major mode previously in effect.

The major mode command should set the variable major-mode to the
major mode command symbol. This is how describe-mode discovers
which documentation to print.

The major mode command should set the variable mode-name to the
"pretty" name of the mode, as a string. This appears in the mode
line.

Since all global names are in the same name space, all the global
variables, constants, and functions that are part of the mode should
have names that start with the major mode name (or with an abbreviation
of it if the name is long). See section Writing Clean Lisp Programs.

The major mode should usually have its own keymap, which is used as the
local keymap in all buffers in that mode. The major mode function
should call use-local-map to install this local map.
See section Active Keymaps, for more information.
This keymap should be kept in a global variable named
modename-mode-map. Normally the library that defines the
mode sets this variable.

The mode may have its own syntax table or may share one with other
related modes. If it has its own syntax table, it should store this in
a variable named modename-mode-syntax-table. See section Syntax Tables.

The mode may have its own abbrev table or may share one with other
related modes. If it has its own abbrev table, it should store this in
a variable named modename-mode-abbrev-table. See section Abbrev Tables.

Use defvar to set mode-related variables, so that they are not
reinitialized if they already have a value. (Such reinitialization
could discard customizations made by the user.)

To make a buffer-local binding for an Emacs customization variable, use
make-local-variable in the major mode command, not
make-variable-buffer-local. The latter function would make the
variable local to every buffer in which it is subsequently set, which
would affect buffers that do not use this mode. It is undesirable for a
mode to have such global effects. See section Buffer-Local Variables.
It's ok to use make-variable-buffer-local, if you wish, for a
variable used only within a single Lisp package.

Each major mode should have a mode hook named
modename-mode-hook. The major mode command should run that
hook, with run-hooks, as the very last thing it
does. See section Hooks.

The major mode command may also run the hooks of some more basic modes.
For example, indented-text-mode runs text-mode-hook as
well as indented-text-mode-hook. It may run these other hooks
immediately before the mode's own hook (that is, after everything else),
or it may run them earlier.

If something special should be done if the user switches a buffer from
this mode to any other major mode, the mode can set a local value for
change-major-mode-hook.

If this mode is appropriate only for specially-prepared text, then the
major mode command symbol should have a property named mode-class
with value special, put on as follows:

(put 'funny-mode 'mode-class 'special)

This tells Emacs that new buffers created while the current buffer has
Funny mode should not inherit Funny mode. Modes such as Dired, Rmail,
and Buffer List use this feature.

If you want to make the new mode the default for files with certain
recognizable names, add an element to auto-mode-alist to select
the mode for those file names. If you define the mode command to
autoload, you should add this element in the same file that calls
autoload. Otherwise, it is sufficient to add the element in the
file that contains the mode definition. See section How Emacs Chooses a Major Mode.

In the documentation, you should provide a sample autoload form
and an example of how to add to auto-mode-alist, that users can
include in their `.emacs' files.

The top-level forms in the file defining the mode should be written so
that they may be evaluated more than once without adverse consequences.
Even if you never load the file more than once, someone else will.

Variable:change-major-mode-hook

This normal hook is run by kill-all-local-variables before it
does anything else. This gives major modes a way to arrange for
something special to be done if the user switches to a different major
mode. For best results, make this variable buffer-local, so that it
will disappear after doing its job and will not interfere with the
subsequent major mode. See section Hooks.

(defun text-mode ()
"Major mode for editing text intended for humans to read.
Special commands: \\{text-mode-map}
Turning on text-mode runs the hook `text-mode-hook'."
(interactive)
(kill-all-local-variables)
(use-local-map text-mode-map) ; This provides the local keymap.
(setq mode-name "Text") ; This name goes into the mode line.
(setq major-mode 'text-mode) ; This is how describe-mode
; finds the doc string to print.
(setq local-abbrev-table text-mode-abbrev-table)
(set-syntax-table text-mode-syntax-table)
(run-hooks 'text-mode-hook)) ; Finally, this permits the user to
; customize the mode with a hook.

The three Lisp modes (Lisp mode, Emacs Lisp mode, and Lisp
Interaction mode) have more features than Text mode and the code is
correspondingly more complicated. Here are excerpts from
`lisp-mode.el' that illustrate how these modes are written.

Functions such as forward-paragraph use the value of the
paragraph-start variable. Since Lisp code is different from
ordinary text, the paragraph-start variable needs to be set
specially to handle Lisp. Also, comments are indented in a special
fashion in Lisp and the Lisp modes need their own mode-specific
comment-indent-function. The code to set these variables is the
rest of lisp-mode-variables.

(make-local-variable 'paragraph-start)
;; Having `^' is not clean, but page-delimiter
;; has them too, and removing those is a pain.
(setq paragraph-start (concat "^$\\|" page-delimiter))
...
(make-local-variable 'comment-indent-function)
(setq comment-indent-function 'lisp-comment-indent))

Each of the different Lisp modes has a slightly different keymap. For
example, Lisp mode binds C-c C-l to run-lisp, but the other
Lisp modes do not. However, all Lisp modes have some commands in
common. The following function adds these common commands to a given
keymap.

Here is an example of using lisp-mode-commands to initialize a
keymap, as part of the code for Emacs Lisp mode. First we declare a
variable with defvar to hold the mode-specific keymap. When this
defvar executes, it sets the variable to nil if it was
void. Then we set up the keymap if the variable is nil.

This code avoids changing the keymap or the variable if it is already
set up. This lets the user customize the keymap.

Based on information in the file name or in the file itself, Emacs
automatically selects a major mode for the new buffer when a file is
visited.

Command:fundamental-mode

Fundamental mode is a major mode that is not specialized for anything
in particular. Other major modes are defined in effect by comparison
with this one--their definitions say what to change, starting from
Fundamental mode. The fundamental-mode function does not
run any hooks; you're not supposed to customize it. (If you want Emacs
to behave differently in Fundamental mode, change the global
state of Emacs.)

Command:normal-mode&optional find-file

This function establishes the proper major mode and local variable
bindings for the current buffer. First it calls set-auto-mode,
then it runs hack-local-variables to parse, and bind or
evaluate as appropriate, any local variables.

If the find-file argument to normal-mode is
non-nil, normal-mode assumes that the find-file
function is calling it. In this case, it may process a local variables
list at the end of the file and in the `-*-' line. The variable
enable-local-variables controls whether to do so.

If you run normal-mode interactively, the argument
find-file is normally nil. In this case,
normal-mode unconditionally processes any local variables list.
See section `Local Variables in Files' in The GNU Emacs Manual, for the syntax of the local variables section of a file.

normal-mode uses condition-case around the call to the
major mode function, so errors are caught and reported as a `File
mode specification error', followed by the original error message.

User Option:enable-local-variables

This variable controls processing of local variables lists in files
being visited. A value of t means process the local variables
lists unconditionally; nil means ignore them; anything else means
ask the user what to do for each file. The default value is t.

Variable:ignored-local-variables

This variable holds a list of variables that should not be
set by a local variables list. Any value specified
for one of these variables is ignored.

In addition to this list, any variable whose name has a non-nilrisky-local-variable property is also ignored.

User Option:enable-local-eval

This variable controls processing of `Eval:' in local variables
lists in files being visited. A value of t means process them
unconditionally; nil means ignore them; anything else means ask
the user what to do for each file. The default value is maybe.

Function:set-auto-mode

This function selects the major mode that is appropriate for the
current buffer. It may base its decision on the value of the `-*-'
line, on the visited file name (using auto-mode-alist), or on the
value of a local variable. However, this function does not look for
the `mode:' local variable near the end of a file; the
hack-local-variables function does that. See section `How Major Modes are Chosen' in The GNU Emacs Manual.

User Option:default-major-mode

This variable holds the default major mode for new buffers. The
standard value is fundamental-mode.

If the value of default-major-mode is nil, Emacs uses
the (previously) current buffer's major mode for the major mode of a new
buffer. However, if the major mode symbol has a mode-class
property with value special, then it is not used for new buffers;
Fundamental mode is used instead. The modes that have this property are
those such as Dired and Rmail that are useful only with text that has
been specially prepared.

Function:set-buffer-major-modebuffer

This function sets the major mode of buffer to the value of
default-major-mode. If that variable is nil, it uses
the current buffer's major mode (if that is suitable).

The low-level primitives for creating buffers do not use this function,
but medium-level commands such as switch-to-buffer and
find-file-noselect use it whenever they create buffers.

Variable:initial-major-mode

The value of this variable determines the major mode of the initial
`*scratch*' buffer. The value should be a symbol that is a major
mode command name. The default value is lisp-interaction-mode.

Variable:auto-mode-alist

This variable contains an association list of file name patterns
(regular expressions; see section Regular Expressions) and corresponding
major mode functions. Usually, the file name patterns test for
suffixes, such as `.el' and `.c', but this need not be the
case. An ordinary element of the alist looks like (regexp .
mode-function).

When you visit a file whose expanded file name (see section Functions that Expand Filenames) matches a regexp, set-auto-mode calls the
corresponding mode-function. This feature enables Emacs to select
the proper major mode for most files.

If an element of auto-mode-alist has the form (regexpfunction t), then after calling function, Emacs searches
auto-mode-alist again for a match against the portion of the file
name that did not match before.

This match-again feature is useful for uncompression packages: an entry
of the form ("\\.gz\\'" . function) can uncompress the file
and then put the uncompressed file in the proper mode according to the
name sans `.gz'.

Here is an example of how to prepend several pattern pairs to
auto-mode-alist. (You might use this sort of expression in your
`.emacs' file.)

This variable specifes major modes to use for scripts that specify a
command interpreter in an `!#' line. Its value is a list of
elements of the form (interpreter . mode); for
example, ("perl" . perl-mode) is one element present by default.
The element says to use mode mode if the file specifies
interpreter.

This variable is applicable only when the auto-mode-alist does
not indicate which major mode to use.

Function:hack-local-variables&optional force

This function parses, and binds or evaluates as appropriate, any local
variables for the current buffer.

The handling of enable-local-variables documented for
normal-mode actually takes place here. The argument force
usually comes from the argument find-file given to
normal-mode.

The describe-mode function is used to provide information
about major modes. It is normally called with C-h m. The
describe-mode function uses the value of major-mode,
which is why every major mode function needs to set the
major-mode variable.

Command:describe-mode

This function displays the documentation of the current major mode.

The describe-mode function calls the documentation
function using the value of major-mode as an argument. Thus, it
displays the documentation string of the major mode function.
(See section Access to Documentation Strings.)

Variable:major-mode

This variable holds the symbol for the current buffer's major mode.
This symbol should have a function definition that is the command to
switch to that major mode. The describe-mode function uses the
documentation string of the function as the documentation of the major
mode.

It's often useful to define a new major mode in terms of an existing
one. An easy way to do this is to use define-derived-mode.

Macro:define-derived-modevariant parent name docstring body...

This construct defines variant as a major mode command, using
name as the string form of the mode name.

The new command variant is defined to call the function
parent, then override certain aspects of that parent mode:

The new mode has its own keymap, named variant-map.
define-derived-mode initializes this map to inherit from
parent-map, if it is not already set.

The new mode has its own syntax table, kept in the variable
variant-syntax-table.
define-derived-mode initializes this variable by copying
parent-syntax-table, if it is not already set.

The new mode has its own abbrev table, kept in the variable
variant-abbrev-table.
define-derived-mode initializes this variable by copying
parent-abbrev-table, if it is not already set.

The new mode has its own mode hook, variant-hook,
which it runs in standard fashion as the very last thing that it does.
(The new mode also runs the mode hook of parent as part
of calling parent.)

In addition, you can specify how to override other aspects of
parent with body. The command variant
evaluates the forms in body after setting up all its usual
overrides, just before running variant-hook.

The argument docstring specifies the documentation string for the
new mode. If you omit docstring, define-derived-mode
generates a documentation string.

A minor mode provides features that users may enable or disable
independently of the choice of major mode. Minor modes can be enabled
individually or in combination. Minor modes would be better named
"Generally available, optional feature modes" except that such a name is
unwieldy.

A minor mode is not usually a modification of single major mode. For
example, Auto Fill mode may be used in any major mode that permits text
insertion. To be general, a minor mode must be effectively independent
of the things major modes do.

A minor mode is often much more difficult to implement than a major
mode. One reason is that you should be able to activate and deactivate
minor modes in any order. A minor mode should be able to have its
desired effect regardless of the major mode and regardless of the other
minor modes in effect.

Often the biggest problem in implementing a minor mode is finding a
way to insert the necessary hook into the rest of Emacs. Minor mode
keymaps make this easier than it used to be.

There are conventions for writing minor modes just as there are for
major modes. Several of the major mode conventions apply to minor
modes as well: those regarding the name of the mode initialization
function, the names of global symbols, and the use of keymaps and
other tables.

In addition, there are several conventions that are specific to
minor modes.

Make a variable whose name ends in `-mode' to represent the minor
mode. Its value should enable or disable the mode (nil to
disable; anything else to enable.) We call this the mode
variable.
This variable is used in conjunction with the minor-mode-alist to
display the minor mode name in the mode line. It can also enable
or disable a minor mode keymap. Individual commands or hooks can also
check the variable's value.
If you want the minor mode to be enabled separately in each buffer,
make the variable buffer-local.

Define a command whose name is the same as the mode variable.
Its job is to enable and disable the mode by setting the variable.
The command should accept one optional argument. If the argument is
nil, it should toggle the mode (turn it on if it is off, and off
if it is on). Otherwise, it should turn the mode on if the argument is
a positive integer, a symbol other than nil or -, or a
list whose CAR is such an integer or symbol; it should turn the
mode off otherwise.
Here is an example taken from the definition of transient-mark-mode.
It shows the use of transient-mark-mode as a variable that enables or
disables the mode's behavior, and also shows the proper way to toggle,
enable or disable the minor mode based on the raw prefix argument value.

Here mode-variable is the variable that controls enabling of the
minor mode, and string is a short string, starting with a space,
to represent the mode in the mode line. These strings must be short so
that there is room for several of them at once.
When you add an element to minor-mode-alist, use assq to
check for an existing element, to avoid duplication. For example:

Each minor mode can have its own keymap, which is active when the mode
is enabled. To set up a keymap for a minor mode, add an element to the
alist minor-mode-map-alist. See section Active Keymaps.

One use of minor mode keymaps is to modify the behavior of certain
self-inserting characters so that they do something else as well as
self-insert. In general, this is the only way to do that, since the
facilities for customizing self-insert-command are limited to
special cases (designed for abbrevs and Auto Fill mode). (Do not try
substituting your own definition of self-insert-command for the
standard one. The editor command loop handles this function specially.)

Each Emacs window (aside from minibuffer windows) includes a mode line,
which displays status information about the buffer displayed in the
window. The mode line contains information about the buffer, such as its
name, associated file, depth of recursive editing, and the major and
minor modes.

This section describes how the contents of the mode line are
controlled. It is in the chapter on modes because much of the
information displayed in the mode line relates to the enabled major and
minor modes.

mode-line-format is a buffer-local variable that holds a
template used to display the mode line of the current buffer. All
windows for the same buffer use the same mode-line-format and
their mode lines appear the same (except for scrolling percentages and
line numbers).

The mode line of a window is normally updated whenever a different
buffer is shown in the window, or when the buffer's modified-status
changes from nil to t or vice-versa. If you modify any of
the variables referenced by mode-line-format (see section Variables Used in the Mode Line), you may want to force an update of the mode line so as to
display the new information.

Function:force-mode-line-update

Force redisplay of the current buffer's mode line.

The mode line is usually displayed in inverse video; see
mode-line-inverse-video in section Inverse Video.

The mode line contents are controlled by a data structure of lists,
strings, symbols, and numbers kept in the buffer-local variable
mode-line-format. The data structure is called a mode line
construct, and it is built in recursive fashion out of simpler mode line
constructs. The same data structure is used for constructing
frame titles (see section Frame Titles).

Variable:mode-line-format

The value of this variable is a mode line construct with overall
responsibility for the mode line format. The value of this variable
controls which other variables are used to form the mode line text, and
where they appear.

A mode line construct may be as simple as a fixed string of text, but
it usually specifies how to use other variables to construct the text.
Many of these variables are themselves defined to have mode line
constructs as their values.

The default value of mode-line-format incorporates the values
of variables such as mode-name and minor-mode-alist.
Because of this, very few modes need to alter mode-line-format.
For most purposes, it is sufficient to alter the variables referenced by
mode-line-format.

A mode line construct may be a list, a symbol, or a string. If the
value is a list, each element may be a list, a symbol, or a string.

string

A string as a mode line construct is displayed verbatim in the mode line
except for %-constructs. Decimal digits after the `%'
specify the field width for space filling on the right (i.e., the data
is left justified). See section %-Constructs in the Mode Line.

symbol

A symbol as a mode line construct stands for its value. The value of
symbol is used as a mode line construct, in place of symbol.
However, the symbols t and nil are ignored; so is any
symbol whose value is void.
There is one exception: if the value of symbol is a string, it is
displayed verbatim: the %-constructs are not recognized.

(stringrest...) or (listrest...)

A list whose first element is a string or list means to process all the
elements recursively and concatenate the results. This is the most
common form of mode line construct.

(symbolthenelse)

A list whose first element is a symbol is a conditional. Its meaning
depends on the value of symbol. If the value is non-nil,
the second element, then, is processed recursively as a mode line
element. But if the value of symbol is nil, the third
element, else, is processed recursively. You may omit else;
then the mode line element displays nothing if the value of symbol
is nil.

(widthrest...)

A list whose first element is an integer specifies truncation or
padding of the results of rest. The remaining elements
rest are processed recursively as mode line constructs and
concatenated together. Then the result is space filled (if
width is positive) or truncated (to -width columns,
if width is negative) on the right.
For example, the usual way to show what percentage of a buffer is above
the top of the window is to use a list like this: (-3 "%p").

If you do alter mode-line-format itself, the new value should
use the same variables that appear in the default value (see section Variables Used in the Mode Line), rather than duplicating their contents or displaying
the information in another fashion. This way, customizations made by
the user or by Lisp programs (such as display-time and major
modes) via changes to those variables remain effective.

Here is an example of a mode-line-format that might be
useful for shell-mode, since it contains the hostname and default
directory.

This section describes variables incorporated by the
standard value of mode-line-format into the text of the mode
line. There is nothing inherently special about these variables; any
other variables could have the same effects on the mode line if
mode-line-format were changed to use them.

Variable:mode-line-modified

This variable holds the value of the mode-line construct that displays
whether the current buffer is modified.

The default value of mode-line-modified is ("--%1*%1+-").
This means that the mode line displays `--**-' if the buffer is
modified, `-----' if the buffer is not modified, `--%%-' if
the buffer is read only, and `--%*--' if the buffer is read only
and modified.

Changing this variable does not force an update of the mode line.

Variable:mode-line-buffer-identification

This variable identifies the buffer being displayed in the window. Its
default value is ("%F: %17b"), which means that it usually
displays `Emacs:' followed by seventeen characters of the buffer
name. (In a terminal frame, it displays the frame name instead of
`Emacs'; this has the effect of showing the frame number.) You may
want to change this in modes such as Rmail that do not behave like a
"normal" Emacs.

Variable:global-mode-string

This variable holds a mode line spec that appears in the mode line by
default, just after the buffer name. The command display-time
sets global-mode-string to refer to the variable
display-time-string, which holds a string containing the time and
load information.

The `%M' construct substitutes the value of
global-mode-string, but this is obsolete, since the variable is
included directly in the mode line.

Variable:mode-name

This buffer-local variable holds the "pretty" name of the current
buffer's major mode. Each major mode should set this variable so that the
mode name will appear in the mode line.

Variable:minor-mode-alist

This variable holds an association list whose elements specify how the
mode line should indicate that a minor mode is active. Each element of
the minor-mode-alist should be a two-element list:

(minor-mode-variablemode-line-string)

More generally, mode-line-string can be any mode line spec. It
appears in the mode line when the value of minor-mode-variable is
non-nil, and not otherwise. These strings should begin with
spaces so that they don't run together. Conventionally, the
minor-mode-variable for a specific mode is set to a non-nil
value when that minor mode is activated.

minor-mode-alist is not buffer-local. The variables mentioned
in the alist should be buffer-local if the minor mode can be enabled
separately in each buffer.

Variable:mode-line-process

This buffer-local variable contains the mode line information on process
status in modes used for communicating with subprocesses. It is
displayed immediately following the major mode name, with no intervening
space. For example, its value in the `*shell*' buffer is
(": %s"), which allows the shell to display its status along
with the major mode as: `(Shell: run)'. Normally this variable
is nil.

Variable:default-mode-line-format

This variable holds the default mode-line-format for buffers
that do not override it. This is the same as (default-value
'mode-line-format).

The variable vc-mode, local in each buffer, records whether the
buffer's visited file is maintained with version control, and, if so,
which kind. Its value is nil for no version control, or a string
that appears in the mode line.

`%' if the buffer is read only (see buffer-read-only); `*' if the buffer is modified (see buffer-modified-p); `-' otherwise. See section Buffer Modification.

%+

`*' if the buffer is modified (see buffer-modified-p); `%' if the buffer is read only (see buffer-read-only); `-' otherwise. This differs from `%*' only for a modified
read-only buffer. See section Buffer Modification.

%&

`*' if the buffer is modified, and `-' otherwise.

%s

The status of the subprocess belonging to the current buffer, obtained with
process-status. See section Process Information.

%t

Whether the visited file is a text file or a binary file. (This is a
meaningful distinction only on certain operating systems.)

%p

The percentage of the buffer text above the top of window, or
`Top', `Bottom' or `All'.

%P

The percentage of the buffer text that is above the bottom of
the window (which includes the text visible in the window, as well as
the text above the top), plus `Top' if the top of the buffer is
visible on screen; or `Bottom' or `All'.

%n

`Narrow' when narrowing is in effect; nothing otherwise (see
narrow-to-region in section Narrowing).

%[

An indication of the depth of recursive editing levels (not counting
minibuffer levels): one `[' for each editing level.
See section Recursive Editing.

A hook is a variable where you can store a function or functions
to be called on a particular occasion by an existing program. Emacs
provides hooks for the sake of customization. Most often, hooks are set
up in the `.emacs' file, but Lisp programs can set them also.
See section Standard Hooks, for a list of standard hook variables.

Most of the hooks in Emacs are normal hooks. These variables
contain lists of functions to be called with no arguments. The reason
most hooks are normal hooks is so that you can use them in a uniform
way. You can always tell when a hook is a normal hook, because its
name ends in `-hook'.

The recommended way to add a hook function to a normal hook is by
calling add-hook (see below). The hook functions may be any of
the valid kinds of functions that funcall accepts (see section What Is a Function?). Most normal hook variables are initially void;
add-hook knows how to deal with this.

As for abnormal hooks, those whose names end in `-function' have
a value that is a single function. Those whose names end in
`-hooks' have a value that is a list of functions. Any hook that
is abnormal is abnormal because a normal hook won't do the job; either
the functions are called with arguments, or their values are meaningful.
The name shows you that the hook is abnormal and that you should look at
its documentation string to see how to use it properly.

Major mode functions are supposed to run a hook called the mode
hook as the last step of initialization. This makes it easy for a user
to customize the behavior of the mode, by overriding the local variable
assignments already made by the mode. But hooks are used in other
contexts too. For example, the hook suspend-hook runs just
before Emacs suspends itself (see section Suspending Emacs).

Here's an expression that uses a mode hook to turn on Auto Fill mode
when in Lisp Interaction mode:

(add-hook 'lisp-interaction-mode-hook 'turn-on-auto-fill)

The next example shows how to use a hook to customize the way Emacs
formats C code. (People often have strong personal preferences for one
format or another.) Here the hook function is an anonymous lambda
expression.

At the appropriate time, Emacs uses the run-hooks function to
run particular hooks. This function calls the hook functions that have
been added with add-hook.

Function:run-hooks&rest hookvar

This function takes one or more hook variable names as arguments, and
runs each hook in turn. Each hookvar argument should be a symbol
that is a hook variable. These arguments are processed in the order
specified.

If a hook variable has a non-nil value, that value may be a
function or a list of functions. If the value is a function (either a
lambda expression or a symbol with a function definition), it is
called. If it is a list, the elements are called, in order.
The hook functions are called with no arguments.

For example, here's how emacs-lisp-mode runs its mode hook:

(run-hooks 'emacs-lisp-mode-hook)

Function:add-hookhook function &optional append local

This function is the handy way to add function function to hook
variable hook. The argument function may be any valid Lisp
function with the proper number of arguments. For example,

(add-hook 'text-mode-hook 'my-text-hook-function)

adds my-text-hook-function to the hook called text-mode-hook.

You can use add-hook for abnormal hooks as well as for normal
hooks.

It is best to design your hook functions so that the order in which they
are executed does not matter. Any dependence on the order is "asking
for trouble." However, the order is predictable: normally,
function goes at the front of the hook list, so it will be
executed first (barring another add-hook call).

If the optional argument append is non-nil, the new hook
function goes at the end of the hook list and will be executed last.

If local is non-nil, that says to make the new hook
function local to the current buffer. Before you can do this, you must
make the hook itself buffer-local by calling make-local-hook
(notmake-local-variable). If the hook itself is not
buffer-local, then the value of local makes no difference--the
hook function is always global.

Function:remove-hookhook function &optional local

This function removes function from the hook variable hook.

If local is non-nil, that says to remove function
from the local hook list instead of from the global hook list. If the
hook itself is not buffer-local, then the value of local makes no
difference.

Function:make-local-hookhook

This function makes the hook variable hook local to the current
buffer. When a hook variable is local, it can have local and global
hook functions, and run-hooks runs all of them.

This function works by making t an element of the buffer-local
value. That serves as a flag to use the hook functions in the default
value of the hook variable as well as those in the local value. Since
run-hooks understands this flag, make-local-hook works
with all normal hooks. It works for only some non-normal hooks--those
whose callers have been updated to understand this meaning of t.

Do not use make-local-variable directly for hook variables; it is
not sufficient.